Phosphors have been utilized for vacuum fluorescent displays (VFD), field emission displays (FED), plasma display panels (PDP), cathode ray tubes (CRT), white light emitting diodes (LED), and the like. In all these usages, it is required to supply an energy to an applicable phosphor to thereby excite it so as to cause it to emit light, and the phosphor is excited by an excitation source having high energy such as vacuum ultraviolet light, ultraviolet light, electron beam, blue light, or the like, such that the phosphor is caused to emit visible light.
However, since phosphors are exposed to the aforementioned excitation sources to resultingly cause a problem of deteriorated luminance during long-term usage, thereby necessitating a phosphor which is free of luminance deterioration. As such, there has been proposed a sialon phosphor as a phosphor exhibiting less luminance deterioration, instead of the conventional silicate phosphor, phosphate phosphor, aluminate phosphor, sulfide phosphor, and the like.
The sialon phosphor is produced by a production process as generally described below. Firstly, there are mutually mixed silicon nitride (Si3N4), aluminum nitride (AlN), calcium carbonate (CaCO3), and europium oxide (Eu2O3) at a predetermined molar ratio, followed by holding for 1 hour at a temperature of 1,700° C. in nitrogen at 1 atm (0.1 MPa), and firing by hot pressing for production (see Patent Document 1, for example).
It has been reported that α-sialon obtained by the process and activated by Eu ion is established into a phosphor which is excited by blue light at 450 to 500 nm and caused to emit yellow light at 550 to 600 nm. However, there have been demanded not only the phosphor which emits yellow light but also phosphors which emit orange light and red light, respectively, for usages such as white LED and plasma display each having an ultraviolet LED as an excitation source. Further, there have been demanded phosphors which emit orange light, red light, and the like, respectively, in a white LED having a blue LED as an excitation source, for an improved color rendering property.
As a phosphor which emits red light, there has been reported an inorganic substance (Ba2−xEuxSi5N8; where x=0.14 to 1.16) obtained by activating a Ba2Si5N8 crystal with Eu, in a scientific literature (see Non-patent Document 1) prior to filing of the present application. There has been further reported a phosphor including, as a host material, a ternary nitride of alkali metals and silicon at various compositions, MxSiyNz (M=Ca, Sr, Ba, Zn; where x, y, and z take various values, respectively), in the second chapter of a publication “On new rare-earth doped M-Si—Al—O—N materials” (see Non-patent Document 2). Similarly, there has been reported MxSiyNz:Eu (M=Ca, Sr, Ba, Zn; where z=2/3x+4/3y), in U.S. Pat. No. 6,682,663 (Patent Document 2).
As other sialon phosphors, nitride phosphors, and oxynitride phosphors, there have been described phosphors including, as host crystals, MSi3N5, M2Si4N7, M4Si6N11, M9Si11N23, M16Si15O6N32, M13Si18Al12O18N36, MSi5Al2ON9, and M3Si5AlON10 (where M is Ba, Ca, Sr, or rare earth element) activated with Eu, Ce, or the like in JP-A-2003-206481 (Patent Document 3) and U.S. Pat. No. 6,670,748 (Patent Document 4), and there have been described therein a phosphor which emits red light and an LED lighting unit utilizing the phosphor.
Among them, SrSiAl2O3N2:Eu2+ and Sr2Si4AlON7:Eu2+ have been known as compounds based on EuaSrbSicAldOeNf. Further, there has been reported a phosphor obtained by activating an Sr2Si5N8 or SrSi7N10 crystal with Ce, in JP-A-2002-322474 (Patent Document 5).
In JP-A-2003-321675 (Patent Document 6), there have been found a description of a phosphor LxMyN(2/3x+4/3y):Z (L is a divalent element such as Ca, Sr, Ba, or the like, and M is a tetravalent element such as Si, Ge, or the like, and Z is an activator such as Eu), and a description that addition of a small amount of Al brings about an effect of restricting afterglow. Further, it has been known that a combination of the phosphor with a blue LED provides a light emitting apparatus for emitting warm color based and slightly reddish white light.
In turn, there has been reported a phosphor configured with various L elements, M elements, and Z elements, as an LxMyN(2/3x+4/3y):Z phosphor, in JP-A-2003-277746 (Patent Document 7). Meanwhile, although JP-A-2004-10786 (Patent Document 8) has described a wide variety of combinations concerning L-M-N:Eu, Z types, it has failed to show an effect of improved emission characteristics in case of adopting specific compositions or crystal phases as host materials.
Although the phosphors represented by those of the aforementioned Patent Documents 2 through 7 have been reported as ones including various different crystal phases as host materials while providing known phosphors for emitting red light, emission luminances of red light have been insufficient insofar as based on excitation by blue visible light. Further, the phosphors have been chemically unstable depending on compositions, thereby exhibiting a problem of durability. Moreover, there have been adopted metals such as Ca, Sr, and the like, or nitride, as starting materials, so that the applicable powder is required to be mixedly obtained in a state where air is to be shut off, thereby exhibiting a problem of productivity.
As the related art of lighting apparatus, there has been known a white light emitting diode based on a combination of a blue light emitting diode element with a blue-light absorbing/yellow-light emitting phosphor, which has been practiced in various lighting usages. Representative examples thereof include those described in JP-2900928 (Patent Document 9) entitled “Light Emitting Diode”, JP-2927279 (Patent Document 10) entitled “Light Emitting Diode”, JP-3364229 (Patent Document 11) entitled “Casting Material for Wavelength Conversion, Production Method Thereof, and Light Emitting Element”, and the like.
The phosphors, which are particularly frequently utilized in these light emitting diodes, are yttrium/aluminum/garnet based phosphors represented by a general formula (Y, Gd)3(Al, Ga)5O12:Ce3+.
However, the white light emitting diode comprising the blue light emitting diode element and the yttrium/aluminum/garnet based phosphor has a feature to emit bluish white light due to lack of a red component, thereby problematically exhibiting deviation in a color rendering property.
Under such circumstances, there has been investigated a white light emitting diode including two kinds of mixed and dispersed phosphors, such that a red component lacking in case of a yttrium/aluminum/garnet based phosphor is compensated for by an additional red-aimed phosphor. Examples of such light emitting diodes include JP-A-10-163535 (Patent Document 12) entitled “White Light Emitting Element”, JP-A-2003-321675 (Patent Document 6) entitled “Nitride Phosphor and Production Method Thereof”, and the like. However, there have been still left problems to be improved concerning color rendering property even by these inventions, thereby necessitating a light emitting diode for solving such a problem. Further, the red-aimed phosphor described in JP-A-10-163535 (the Patent Document 12) includes cadmium, thereby exhibiting a problem of environmental pollution. Contrary, although the red-light emitting phosphors including Ca1.97Si5N8:Eu0.03 described in JP-A-2003-321675 (the Patent Document 6) as a representative example do not include cadmium, the phosphors are low in luminance, thereby still necessitating a further improvement of emission intensities thereof.